1. Field of the Invention
The invention relates to a method for operating a CT (computed tomography) device and a computed tomography device for performing a dynamic CT examination on a patient, the computed tomography device being of the type having a gantry with a stationary part and a part that can be rotated around a system axis, with an x-ray radiation source and an x-ray radiation detector disposed opposite one another on the rotatable part, and a patient support plate that can be moved in the direction of the system axis. The invention also relates to a non-transitory data storage medium, on which program code is encoded that implements such a method.
2. Description of the Prior Art
In addition to conventional CT examinations, in which slice images or 3D images of a body region of a patient are reconstructed to obtain information about the morphology of the patient, so-called dynamic CT examinations are now established procedures, that are used to obtain functional information, for example about patient tissue. Contrast agents are frequently used in this type of computed tomography.
An example of such dynamic examination is a multiphase examination of the liver of a patient, of which images are produced in different time phases or different states, in order to be able to distinguish between different types of lesions in the liver for diagnostic purposes. In the case of the liver the different phases or states are produced by administering contrast agent, which is absorbed by the different types of lesions at different times. The multiphase examination of the liver therefore includes a so-called native phase, in which no contrast agent is present in the liver, a second arterial phase after the administration of contrast agent and a third venous phase after the administration of contrast agent, following the arterial phase. In order to be able to reconstruct images in all the liver phases, it is necessary to record x-ray projections of the body region containing the liver in all the liver phases over quite a long time period of approx. 30 to 50 seconds, for example to evaluate perfusion parameters.
According to a first method the body region of the patient containing the liver is positioned with the patient support plate in the measurement volume of the computed tomography device defined by the x-ray radiation source and the x-ray radiation detector and, with the patient support plate stationary, successive x-ray projections are recorded of the body region of the patient containing the liver, for image reconstruction purposes. A disadvantage of this recording technique is that the examinable body region is limited to the width of the x-ray radiation detector when viewed in the direction of the system axis or the longitudinal axis of the patient, and this cannot easily be extended, at least with existing computed tomography devices. Arbitrary patient movement and respiratory movement make it desirable to have greater coverage in the longitudinal direction of the patient when recording x-ray projections. FIG. 1 shows the described situation, with the width of the x-ray radiation detector or the extension A1 of an x-ray projection PR in the direction of the system axis SY defining the scan region S1 in the direction of the system axis SY or the body region of the patient P to be examined. The dose profile D1 of the dose of x-ray radiation applied to the patient P during the recording of x-ray projections is relatively homogeneous and is also based on the width of the x-ray radiation detector or the extension A1 of the x-ray projections PR in the direction of the system axis SY.
To avoid the disadvantages of the first method, a second method was created, in which while the measurement system remains otherwise the same, but during the recording of x-ray projections the patient support plate bearing the patient is moved forward and back periodically and continuously when viewed in the direction of the system axis or the longitudinal axis of the patient within a scan region S2, so that x-ray projections PR of a longer body region of the patient can effectively be recorded. Compared with the first method, the dose profile D2 of the dose of x-ray radiation applied to the patient during the recording of x-ray projections PR widens. The distribution of the dose when viewed in the direction of the longitudinal axis of the patient is however comparatively homogeneous. FIG. 2 shows the method, with which a larger or longer body region of the patient can be examined by moving the patient support plate holding said patient, while the x-ray radiation detector remains in a fixed position.
Computed tomography devices are now being used that have a wider x-ray radiation detector when viewed in the direction of the system axis than previously used computed tomography devices. While some years ago so-called 16-slice detectors were still the standard for x-ray radiation detectors, the standard is now 64-slice detectors, or x-ray radiation detectors with even more slices. In the case of wider x-ray radiation detectors, assuming that the scan region does not get longer, since the anatomy of the patient does not change, the shape of the dose profile of the dose of x-ray radiation applied to the patient during the recording of x-ray projections changes. Particularly in the case of a relatively short scan region compared with detector coverage, there is a clear rise in the dose of x-ray radiation in the central body section of the body region of the patient to be examined, which is exposed almost continuously to x-ray radiation despite the movement of the patient support plate, without the additionally obtained information being necessary for diagnostic purposes. FIG. 3 illustrates the problem. While scan region S3 corresponds to scan region S2 from FIG. 2, the width of the x-ray radiation detector or the extension A3 of an x-ray projection PR in the direction of the system axis is much larger than the width of the x-ray radiation detector or the extension A1 of an x-ray projection PR in the direction of the system axis from FIG. 2. Despite the movement of the patient support plate, the body section of the patient to be assigned to the center of the scan region is permanently exposed, so the dose profile D3 results with a clear rise in the dose of x-ray radiation in the central region.